1. Field of the Invention
The present invention relates to superplastic aluminum alloys and particularly to an improved method for producing such materials.
2. Description of the Prior Art
Efforts to produce improved superplastic aluminum alloys, i.e., alloys of aluminum which can be superplastically formed using gas pressure or vacuum have been numerous and extensive as evidenced by the plethora of prior art describing such materials and methods for their preparation.
Among this prior art, two relatively recent techniques appear to produce commercially valuable superplastic materials.
The first of these techniques is described in U.S. Pat. No. 3,847,681 issued Nov. 12, 1974, to Waldman et al. This technique, which is presented schematically in FIG. 1 hereof, involves the steps of:
(a) solution heat treating the starting material for from 4-48 hours at a temperature greater than 860.degree. F.;
(b) slow cooling the product of step (a) to an overage temperature, i.e., about 775.degree. F.;
(c) overaging at about 775.degree. F. for 3 to 6 hours;
(d) slow cooling the product of step (c) to a temperature of between about 450.degree.-500.degree. F. and optionally holding at this temperature for up to 4 hours;
(e) plastically deforming the material (from 40-80%) at a temperature between about 450.degree. and 500.degree. F.; and
(f) rapidly recrystallizing at a temperature of between about 800.degree. and 900.degree. F.
This process reportedly provides a fine grain structured 7000 series alloy.
Photomicrographs of 7475 alloy prepared using this procedure demonstrate that, although the grains are relatively fine, their aspect ratio, i.e., length to width ratio, is quite high.
The second prior art process which produces acceptable material is that described in U.S. Pat. No. 4,092,181 issued May 30, 1978 to Paton et al. This patent describes a process for preparing material reportedly of finer grain than that described in the U.S. Pat. No. 3,847,681, according to a somewhat shorter procedure, and in heat treatable alloys other than those of the U.S. Pat. No. 3,847,681, which additional alloys may include chromium as an alloying element.
The process of the U.S. Pat. No. 4,092,181 is quite similar to that of the U.S. Pat. No. 3,847,681 except that it offers the option of cold water quenching after solution heat treat and before overage (i.e., between steps (a) and (c) of U.S. Pat. No. 3,847,681) and eliminates the need entirely for the optional soaking or holding of step (d) of the U.S. Pat. No. 3,847,681.
In each of these references, the mechanical work required to induce the lattice strain necessary for recrystallization is performed while the material is warm, i.e., at between 400.degree. and 650.degree. F. Although the U.S. Pat. No. 4,092,181 alludes to the feasibility of performing such work at lower temperatures, i.e., "below the overage temperature" there is no disclosure of "cold" rolling, i.e., rolling at room temperature.
Both of the foregoing processes can provide useful superplastic materials under some conditions as evidenced by evaluation thereof by the inventors of the present process. These prior art processes are, however, somewhat difficult to work into a commercial production operation because of the apparent requirement that the material be subjected to an overaging step requiring holding the same at a temperature of from about 700.degree. F. to about 800.degree. F. for a period of from about 2 to about 8 hours before cooling to a lower temperature to plastically reduce the metal. The pressure of such a step in the manufacturing operation makes it essentially a batch-type process which requires substantial material handling and does not take advantage of the obvious economics inherent in a continuous rather than batch process system. The reasoning for this statement is that plastic deformation is done below the recovery temperature and the metal is limited in the percent reduction it can take. By hot/warm rolling at the temperatures as taught in this patent the metal can be reduced a much greater amount, thus allowing reduction to be taken at a much greater starting thickness. Avoiding the necessity of rolling relatively thin starting slabs (.ltoreq.1.5") allows the metal to be processed into a coil which is more amenable to a continuous processing operation.
The advantages of fine and equiaxed grain structure in superplastic materials are discussed in detail in "Superplasticity", J. W. Edington, K. N. Melton and C. P. Cutler, Progress in Materials Science, Vol. 21, No. 2, pp. 63-170, Pergammon, New York (1976).